Method and apparatus for co{11 {11 conversion to methane

ABSTRACT

A process of fixation and conversion of carbon dioxide from the atmosphere or other sources to produce methane and oxygen. Carbon dioxide is scrubbed from a CO2-containing source and separated by process of chemical concentration. A special cell is provided in which hydrogen is produced and reacted with the separated CO2 at methanation conditions to produce methane.

l nited States Patent [191 Eregory METHOD KND' APPKRATUSFOK C02 FOREIGN PATENTS OR APPLICATIONS CONVERSION To METHANE 5 069 9/ 1902 Denmark 204/76 [75] Inventor: Derek P. Gregory, Hinsdale, lll. I I I I I l [73] Assignee: Institute of Gas Technology, Prim ry X mine C- dmundSOn Chicago, Ill. Att0rney-A. W. Molinare et al.

[22] Filed: Feb. 4, 1972 [21] Appl. No.: 223,546 [57] ABSTRACT A process of fixation and conversion of carbon dioxide from the atmosphere or other sources to produce [52] U.S.(il. 204/72, 204/76 methane and oxygen Carbon dioxide is scrubbed from .n a cor i i g Source d separated process of 1 of care 204,73 76 chemical concentration. A special cell is provided in which hydrogen is produced and reacted with the sep- [56] UNITE S ES QZiENTS arated CO at methanation conditions to produce methane.

1,431,047 10/1922 Ruben 204/76 1,730,997 10/1929 Danckwardt 204/73 16 Clam, 1 Drawmg 8 D. C POWER SOURCE DE- ZSORPTION 6. 5012mm ELECTROLYSB' 16 METHANATION 'l l cm. 1 5

SOURCE SEPARATlONfl fiQ SEPARATION Patented Oct 16', 1973 3,766,027

D. C. POUE R SOU CE ELECTROLYSFS' EANA'HON E LL , l METHOD AND APPARATUS FOR CO2 CONVERSION TO MlE'lIIANE BACKGROUND OF THE INVENTION cal cell which combines the production of purge hydrogen with methanation.

Depletion of fossil hydrocarbons is occurring at increasingly rapid rates from two, competing needs of mankind. The major depletion is due to the use of fossil hydrocarbons as fuels which are burned. The competing use for the fossil hydrocarbons is for feed stocks for the organic chemical industry.

Thus, the depletion of the fossil hydrocarbons endangers not only the energy supplies, but also the basic building blocks for numerous end products, principally plastics, paints, solvents, drugs, pharmaceuticals, detergents, rubbers and the like.

In addition, as the fossil fuels are burned, the level of carbon dioxide in the atmosphere tends to rise. In turn, the increase in CO content leads to heating the earths surface as a result of the greenhouse effect. In confined areas, such as submarines or spacecraft unless precautions are taken to prevent it, excessive carbon dioxide levels develop which are harmful to life.

OBJECTS OF THE INVENTION It is, therefore, an object of this invention to provide a process which can help to maintain the level of CO in the atmosphere or other sources at an acceptable level, and convert it to methane as a basic fuel source of energy, and as a feed stock for industrial processes for production of a wide range of organic chemicals.

It is another object of this invention to recover carbon dioxide from the atmosphere or other sources such as combustion flue gases.

It is another object of this invention to conserve the world supply of fossil hydrocarbons as fuel and industrial process feed stocks.

It is another object of this invention to provide a process for the fixation of atmospheric or other carbon dioxide coupled with'a process for production of hydrogen followed by methanation of the scavenged carbon dioxide.

Still other objects will be evident from the description of the invention which follows:

SUMMARY OF THE INVENTION In the process of this invention, carbon dioxide is scrubbed from the atmosphere or other sources by a carbon dioxide absorber, from which the carbon dioxide is then liberated by regeneration with heat, and swept out of the regenerator with a stream of purge hydrogen or methane. The hydrogen is produced in a special electrolysis cell having an electrolyte compatible with CO and operated at methanation conditions, with the hydrogen evolving electrode being fed with the CO evolved from the regeneration stage of the absorber. The conditions in the cell are controlled to produce methane. In turn this lowers hydrogen pressure and depolarizes the cell. The methanation zone of the cell is preferably in heat exchange relationship with the regeneration phase of the CO absorber to provide the heat necessary for regenerating the CO absorbing liquid.

The end product methane is recovered and may be utilized as a natural gas substitute, for example peakshaving gas, but is also useful as a feed stock for the production of more valuable chemical compounds such as in reforming processes to produce carbon monoxide and hydrogen, or methanol, or other organic chemicals.

This process is particularly adaptable to the use of nuclear energy to provide the electricity for the electrolyzer, and thus this process provides a ready source for methane fuel and feed stocks as fossil hydrocarbons become more scarce.

Detailed description of the preferred embodiment follows having reference to:

The FIGURE which is a schematic representation of the process of this invention.

The detailed description and preferred embodiment are by way of illustration and not by limitation, it being understood that variations in the invention may be employed withoutdeparting from the spirit thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The normal carbon dioxide levels in the earth s atmosphere are in the range of 300 to 500 ppm (0.03 percent 0.05 percent by volume) with oxygen being present in the range of 20 percent to 21 percent. The carbon dioxide scrubbed from the air must be relatively free from oxygen since it is to be reacted with hydrogen at relatively high temperatures. These conditions present the possibility of combustion of any residual oxygen and hydrogen, and must be prevented. Accordingly, our process employs carbon dioxide relatively free of oxygen which refers to maintaining the oxygen concentration below the explosive limits of the carbon dioxide-hydrogen mixture produced for reaction in the methanation stage of this process. The carbon dioxide produced in accord with our process thus contains less than about 15 percent oxygen. In order to reduce the wasteful loss of hydrogen by reaction with this residual oxygen on the catalyst, it is preferred to keep the oxygen concentration to a minimum.

Although nitrogen contamination is not ,a critical problem since it is relatively inert at the subsequent methanation conditions, it is preferred to keep the nitrogen concentration to a minimum to avoid adverse dilution effects in the methanation processing stage.

In the first step of the process in accord with this invention, carbon dioxide is separated from air or other sources such as combustion process flue gases by any conventional means such as cryogenic fractional distillation, semi-permeable membrane separators, or selective chemical sorption.

It is preferred in this invention to use the chemical techniques utilizing carbon dioxide selective materials since they generally work at relatively low pressures and ambient temperatures, and the concentration of carbon dioxide in the air is relatively small. For cryogenic fractional distillation and the semi-permeable membrane separators, vast quantities of air must be either compressed or moved against pressure differentials in order to effect separation which is expensive in terms of energy.

The chemical sorption processes are relatively inexpensive in terms of energy requirements, and are particularly suitable since most can be regenerated. Suitable carbon dioxide selective chemical sorbents include solids such as molecular sieves, and liquids such as aqueous solutions of sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, and organic liquids such as mono, di-, and tri-ethanol amines, diamines, isopropanol, and mixtures thereof. A preferred sorbent used in this invention is monoethanolamine.

The exact conditions at which carbon dioxide is to be sorbed varies with a given sorbent. Temperatures in the range of to 750 F, depending on the sorbent, may be employed. In the particular instance of monoethanolamine, air and liquid amine are countercurrently contacted at a temperature in the range of from about 50 to 200 F, preferably 100 to 120 F. Other CO absorbing processes may be used, particularly the Purasol process using a methyl-2-pyrrolidone sorbent, or the Rectasol process using methanol at low temperatures as a solvent.

Unlike the usual prior art problem of air-scrubbing which has been faced for example in connection with alkaline fuel cell input stream scrubbing, it is not necessary to remove all of the CO from the input air. Rather, in the process of this invention, as much carbon dioxide as possible is extracted in the absorber, and to regenerate the absorber in a highly concentrated form without contamination by oxygen. The process of this invention works essentially in the reverse of the usual scrubbing process in that the air exhausting from the scrubber is not used in any process but is ejected back to the atmosphere unused, whereas the fixed carbon dioxide absorbed by the scrubber absorbent is then recovered and utilized as the process gas stream. Whereas in prior art processes it is not important as to the type of purge gas used to desorb the carbon dioxide from the absorbent material, this becomes critical in our process since oxygen contamination with the carbon dioxide in a concentration within the combustion limits of the subsequent carbon dioxide-hydrogen mixture leads to dangerous conditions.

In the process of this invention, the carbon dioxideladen sorbent is regenerated by the combination of a gaseous desorbent purge and heat. The heat is provided by heat exchange with the exothermic methanation zone of the special cell, which is described in more detail below. It is preferred to regenerate the sorbent such as monoethanolamine containing the carbon dioxide absorbed therein by contacting the CO laden sorbent with hydrogen or a mixture of hydrogen and methane since these compounds are present in the methanation zone in the special cell, and need not be separated from the carbon dioxide before that subsequent processin. In addition, the hydrogen in this process, being derived from an electrolytic cell is substantially oxygen free and will not introduce contaminant oxygen to the carbon dioxide.

As with the sorption, suitable regeneration (desorption) conditions vary with the sorbent. Temperatures vary with the sorbent used, and are generally higher than the sorption step, generally in the range of from 100 to 1,000 F. In the particular instance of the preferred sorbent, monoethanolamine, temperatures in the range of from about 250 to 400 F, preferably 300 to 350 F, and liquid to purge gas, for example, hydrogen, volume ratios of 4H to ICC; are desirable to produce a stoichrometric mixture for methane production.

Hydrogen is obtained relatively free of oxygen by the electrolysis of water in an acid electrolysis zone utilizing suitable acid electrolytes. Like the carbon dioxide, the hydrogen must be relatively free of oxygen (below the combustion limit). Preferably the hydrogen contains less than 2 percent by volume 0 The by-product oxygen formed by the electrolysis step has utility in a broad application of industrial processes such as steel production, advanced sewage treatment, and water recycling processes.

The desorbent stream coming from the desorption zone is then passed into a methanation zone in the cell wherein the conditions are adjusted for a methanation reaction between carbon dioxide and hydrogen to produce methane and water vapor. The water vapor is subsequently separated from the methane stream and the methane end product is utilized as above described. This methanation reaction is an exothermic one and the heat evolved is recovered by conventional heat exchange means and utilized to supply heat to the regeneration stage of the CO scrubber to desorb the carbon dioxide from the chemical sorbent.

The methanation reaction between carbon dioxide and hydrogen is a known and established process and produces as products methane and water as well as carbon monoxide, an intermediate in the reaction. The methanation reaction product can be separated to produce a pure methane stream and the excess hydrogen and/or carbon dioxide if any, rcycled. Similarly, carbon monoxide, if present, may be recycled to the reaction zone or withdrawn as a separate product steam for conversion to other chemicals such as methanol.

in more detail, the electrolysis reaction and methanation reaction are combined in an electrolysis cell compatible with carbon dioxide to produce the methane in a single step. Suitable electrolytes for such an electrolysis cell include mineral acids, such as phosphoric acid, carbonates such as alkali metal carbonates (Li CO K CO and Na CO and solid oxide electrolytes such as a stabilized zirconia. The cell is operated at conditions under which methanation takes place, for example in a temperature range of about 250 to 2,000 F, preferably 400 1,000 F, with the hydrogen evolving electrode being fed with the carbon dioxide. It may be preferable to operate a cell under pressure to prevent evaporation of electrolyte at these temperatures. The hydrogen is converted immediately upon evolution at the electrode, which results in lowering the hydrogen partial pressure and depolarizing the cell. Further, the heat released in the methanation reaction will lower the cell voltage and decrease the electrical energy input required.

In this single stage process, a reaction product comprising methane and water will be produced along with intermediate products such as carbon monoxide and unconverted hydrogen and/or carbon dioxide. At least a portion of the methane so produced, before or after further separation, is preferably utilized to purge or regenerate the carbon dioxide containing sorbent utilized in the air separation step to recover the carbon dioxide for conversion in the combination electrolysismethanation cell.

The overall reaction in the methanation electrolysis cell may be represented as follows: 4H O( l) CO (g) CH (g) 2H O(g) 204g); A H +233.7 Kc al 5 This represents an energy saving of about l lpercent over a two-step process involving hydrogen production and methanation in separate zones. Further, in another embodiment of this invention, the heat generated by electrolysis inefficiency is recovered and used as heat in the carbon dioxide desorption step.

OPERATION OF THE PROCESS The FIGURE shows in schematic diagram form the fixation of carbon dioxide from the atmosphere or any other convenient source, and the production of methane therefrom by reaction with electrolytic hydrogen in a single stage. Miscellaneous appurtenances are eliminated from the FIGURE with only those lines and zones necessary for a complete and clear understanding of the process of the present invention being illustrated and discussed.

Referring to the FllGURE, a gas su ch as air, contain ing about 400 ppm carbon dioxide is passed into absorption zone 2 via line l where the carbon dioxide is selectively sorbed on a carbon dioxide selective sorbent material. For example, the gas is cocurrently or counter-currently bubbled through liquid monoethanol amine entering zone 2 via line 5. The sorption is main tained at temperatures substantially in the range of from 50 to 200 F. The carbon dioxide-depleted gas is exhausted from the sorption zone 2 via line 3 and discharged to the atmosphere. The CO -saturated sorbent material is passed into desorption zone 6 via line 4i. Fresh sorbent material enters zone 2 via line 5 from the desorption zone as described in more detail below.

in desorption zone a, the carbon dioxide contained in the selective sorbent is removed by purging with a hereinafter described methane or hydrogen-containing stream entering via line R9 to produce a regenerated carbon dioxide selective sorbent which is removed via line 5 for passage to sorption zone The resultant methane-carbon dioxide mixture is removed from desorption zone 6 via line '7, and passed to electrolysismethanation zone h. i

The desorption zone is maintained at a temperature in the range of from about 250 to 400 1F by heat exchange 16 with the special electrolysis-methanation zone d. The hydrogen or methane-containing gas purges the carbon dioxide-containing desorbent regenerating it for recycle to the sorption zone 2 via line 5. As illustrated, sorption zone 2?, and desorption zone 6 are separate zones, however, in a commercial embodiment these separate zones may be contained within a single zone.

The special electrolysis-methanation zone ti is part of an'electrolysis cell, e.g., an acid electrolysis cell operating at from 250 to 450 F, wherein water entering via line 9' is electrolyzed by cathode ill and anode 112 to produce hydrogen and oxygen respectively. The negative current for cathode till is preferably provided by a nuclear powdered DC power source 113 via line M. Similarly, the positive current for anode 112 is provided by line 115. Within electrolysis-methanation zone t5, the

carbon dioxide contained within line 7 is contacted with the hydrogen as it is evolved from cathode 111, thereby effecting an almost simultaneous conversion of the carbon dioxide and hydrogen to methane. A portion of the carbon dioxide and hydrogen may be converted to carbon monoxide under the methanation zone conditions, but this can be converted to methane if desired. Oxygen evolved by anode i2 is removed from electrolysis zone d via line llti for further industrial uses.

The reaction product from methanation zone 8 comprising methane, water vapor, unconverted hydrogen, if any, unconverted carbon dioxide, if any, and intermediate product carbon monoxidc, if any, are removed via line ]l7 and passed to separation zone T8 wherein water is removed via line Zll as above described. This is substantially pure and may be recycled to electrolysis cell via line 9 in one embodiment of our process. A relatively water-free mixture containing the gas enumerated just above is removed from separation zone it; via line 20. This product gas mixture may be further processed in separation zone 22 to provide a recycle hydrogen stream 23, a recycle carbon dioxide stream 241, a recycle or product CO stream 22 and a product methane stream 26. Any one or more of the CO, H and/or Ct) streams 22, 23, 24 may be recycled directly to the desorption zone ti via line 19, to assist in the purge, or passed into the methanation zone via lines '7 or 27 for reaction.

The carbon monoxide present in the stream l7 exiting from the methanation zone is due to only partial reaction of the carbon dioxide with the hydrogen. This carbon monoxide stream can be recycled to the methanation zone via lines 7 or 27 to complete its reaction. In addition, the methane in line ll'7 may be separated from the mixture of other gases and these other gases,

carbon monoxide, hydrogen and residual CO recycled as a combined stream back to the desorption or the methanation zones as above described. Likewise, a portion of the methane may be used to purge the desorption zone 6. Where desired, the carbon monoxide may be used directly as a product stream such as for example in the production of methanol or carbon black.

From the foregoing description it is readily seen by those skilled in the art that there has been provided a process which can readily fix and convert atmospheric carbon dioxide and electrolytic hydrogen to methane in a special cell. This process offers a method for removing carbon dioxide, from the atmosphere and producing a highly useful chemical, methane, which can be used as a fuel or converted to other chemicals and thus conserve our fossil hydrocarbon fuel supply.

it should be understood that while reference has been made to atmospheric air as one source of CO any convenient CO source may be used. For example, some process flue gas streams, for example power plant combustion processes may be relatively rich in CO and thus could be used as the CO source. Many other variations in the scope of the process of this invention may be made without departing from the spirit thereof.

ll claim:

ll. A process for the conversion of carbon dioxide to methane which comprises the steps of:

a. passing a gas containing carbon dioxide intoicontact with a carbon dioxide selective sorbent at sorption conditions to produce a carbon dioxidecontaining sorbent relatively free of oxygen,

b. contacting the carbon dioxide-containing sorbent with a relatively oxygen-free gaseous stream at desorption conditions to produce a relatively oxygent'ree mixture of carbon dioxide-containing gas, and to regenerate said sorbent for reuse as said sorbent in step (a),

c. passing said relatively oxygen-free carbon dioxidecontaining gas into an exothermic electrochemical reaction zone having a hydrogen evolving electrode,

d. maintaining said reaction zone at electrolysis and methanation conditions to produce hydrogen,

e. reacting said relatively oxygen-free carbon dioxide-containing gas in said reaction zone with said hydrogen to produce a gas mixture containing methane, and

f. recovering methane from said methane-containing mixture.

2. A process of claim 1 wherein the carbon dioxide selective sorbent is selected from a. molecular sieves,

b. aqueous solutions of sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate,

c. monoethanolamine, diethanolamine, triethanolamine, methyl-2-pyrrolidone,

d. diamino isopropanol, methanol and e. mixtures thereof.

3. A process of claim 2 wherein the carbon dioxide selective sorbent is selected from mono-, di-, and triethanolamine, methyl-2-pyrrolidone, and methanol.

4. A process of claim 1 which includes the added step of transferring the exothermic heat of reaction produced in said reaction zone to said desorption zone to selectively effect desorption.

5. A process as in claim 3 which includes the steps of:

g. passing said carbon dioxide-containing sorbent produced in step (a) into a desorption zone to effect desorption therein, and

h. recycling said regenerated sorbent into contact with said air to effect resorption of carbon dioxide.

6. A process as in claim 1 which includes the step of:

g. passing a portion of said methane-containing gas mixture to the desorption zone as said oxygen-free gaseous stream to effect said desorption.

7. A process as in claim 1 wherein said step (f) of methane recovery includes the step of separation of water from said methane-containing gas mixture, and which includes the added step of:

g. recycling said water to said electrochemical reaction zone.

8. A process as in claim 1 wherein said step (f) of methane recovery from said methane-containing gas mixture includes separation therefrom of residual CO H and CO produced in said methanation zone.

9. A process as in claim 8 wherein said residual CO and H are recycled to said methanation zone.

10. A process as in claim 8 wherein said residual CO and H are recycled to said desorption step (b) as said oxygen-free gaseous desorption stream.

11. A process as in claim 1 wherein said desorption step (b) is maintained at a temperature within the range of from about to 1,000 F.

12. A process as in claim 1 wherein said electrolysis methanation step (d) is maintained at a temperature within the range of from about 250 to 2,000 F.

13. A process as in claim 1 which includes the step of:

g. passing said recovered methane to said desorption zone to effect said desorption.

14. A process as in claim 1 wherein said reaction zone utilizes an electrolyte selected from an acid, a carbonate or a solid oxide.

15. A process as in claim 1 wherein said gas is air.

16. A process as in claim 1 wherein said gas is process flue gas relatively rich in CO 

2. A process of claim 1 wherein the carbon dioxide selective sorbent is selected from a. molecular sieves, b. aqueous solutions of sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, c. monoethanolamine, diethanolamine, triethanolamine, methyl-2-pyrrolidone, d. diamino isopropanol, methanol and e. mixtures thereof.
 3. A process of claim 2 wherein the carbon dioxide selective sorbent is selected from mono-, di-, and triethanolamine, methyl-2-pyrrolidone, and methanol.
 4. A process of claim 1 which includes the added step of transferring the exothermic heat of reaction produced in said reaction zone to said desorption zone to selectively effect desorption.
 5. A process as in claim 3 which includes the steps of: g. passing said carbon dioxide-containing sorbent produced in step (a) into a desorption zone to effect desorption therein, and h. recycling said regenerated sorbent into contact with said air to effect resorption of carbon dioxide.
 6. A process as in claim 1 which includes the step of: g. passing a portion of said methane-containing gas mixture to the desorption zone as said oxygen-free gaseous stream to effect said desorption.
 7. A process as in claim 1 wherein said step (f) of methane recovery includes the step of separation of water from said methane-containing gas mixture, and which includes the added step of: g. recycling said water to said electrochemical reaction zone.
 8. A process as in claim 1 wherein said step (f) of methane recovery from said methane-containing gas mixture includes separation therefrom of residual CO2, H2 and CO produced in said methanation zone.
 9. A process as in claim 8 wherein said residual CO2 and H2 are recycled to said methanation zone.
 10. A process as in claim 8 wherein said residual CO2 and H2 are recycled to said desorption step (b) as said oxygen-free gaseous desorption stream.
 11. A process as in claim 1 wherein said desorption step (b) is maintained at a temperature within the range of from about 100* to 1,000* F.
 12. A process as in claim 1 wherein said electrolysis methanation step (d) is maintained at a temperature within the range of from about 250* to 2,000* F.
 13. A process as in claim 1 which includes the step of: g. passing said recovered methane to said desorption zone to effect said desorption.
 14. A process as in claim 1 wherein said reaction zone utilizes an electrolyte selected from an acid, a carbonate or a solid oxide.
 15. A process as in claim 1 wherein said gas is air.
 16. A process as in claim 1 wherein said gas is process flue gas relatively rich in CO2. 